Volker Mohrholz (IOW , Germany); Anna Bulczak (IOPAN, Poland); Peng Feng (Chalmers University of Technology, Sweden); Jiaming Chen (University of Bonn, Germany); Marco Rivera (University of Bonn, Germany); Faruque Abdulla (University of Bonn, Germany); Jaromir Jakacki (IOPAN, Poland); Christopher Buckhaupt (CISESS/UMD, College Park, Maryland , US ); Ulf Graewe (IOW, Germany); Jürgen Kusche (University of Bonn, Germany)

The project CONWEST-DYCO2 continues project CONWEST-DYCO in the Elbe estuary and Baltic Sea and extends the analysis to two other two regions. The central hypothesis is that hydrodynamic processes of the river-to-ocean continuum and of the mesoscale in coastal zone are today at best resolved and understood by combining SWOT and SAR-satellite altimetry observations. The main objectives is to evaluate the contribution of SWOT to analyse coastal processes. Our focus is on small scale processes in the transition zone from land to sea, that is in estuaries and in coastal zone.

For this purpose, the SWOT data products are analysed separately and then merged to data of nadir-altimeters in 2-dimensional maps with measurements from other satellite altimeter missions, and will be analyzed against in-situ and modeling results.

In this work we investigate the data quality and the temporal evolution of fine scale structures along coast and estuaries of the Baltic Sea in the time interval from April 2023 to December 2024. For this scope we have constructed daily maps of sea level height above a reference mean sea surface by merging data of Sentinel-3 and Sentinel-6 SAR nadir- with and without SWOT wide-swath altimeter missions using an Optimal Interpolation (OI) algorithm. The geostrophic ocean currents are derived from the maps of absolute and relative dynamic topography. The altimeter heights and few selected environmental corrections are validated at few in-situ locations.

The nadir-altimeter mission data along-track have been processed with unfocused (UF-SAR) and focused (FF-SAR) techniques to increase the data density near the coast. For each gauge an altimeter time-series is constructed by selecting for each cycle the nearest altimeter point to the gauge at a minimum distance to the coast. The mean of the standard deviation difference of the pair of time-series is larger than 10 cm for all the Baltic Sea sub-regions. The FFSAR and the LSAR (UF-SAR) reduced at the same frequency of 20 Hz are comparable. The SWOT data product LR L3 Expert data version 2.0.1. in a 2 km x 2 km grid have been selected. For each gauge an altimeter time-series is constructed by selecting for each cycle the nearest SWOT point to the tide gauge at a minimum distance to the coast. The mean of the standard deviation difference of the pair of time-series is smaller than 10 cm for all the Baltic Sea sub-regions. This indicated that the gridded SWOT products are more accurate than the along-track nadir-altimetry. The time-series are smoother.

Daily maps are constructed by merging nadir-altimeters and SWOT data over the cal/val and science phase. Several OI input parameters are used to best match the fine spatial scales observed by SWOT and the less fine spatial scales present in the CMEMS maps. Compared to in-situ data, the accuracy on nadir-only maps is lower than for the CMEMS grids and larger than 10 cm. Conversely, a better agreement with in-situ is obtained for the daily merged grids, which variability is higher than the variability of the standard CMEMS grid products. Along-track data are assimilated in a CEMS regional ocean model run at IOPAN. The IOW ocean model is further used for comparison.

Changes relative to land and coastal impacts are studied in the three key areas of the Kattegat-Danish Straits, Baltic Proper and Gdansk Basin. In the Gotland Basin we follow the mesoscale circulation structure observed between April 18 and April 28 during the SWOT-ADAC campaign showing the good agreement with the steric heights from the CTDs measurements.